Resistance-type sensors are known for measuring the concentration of a combustible gas component such as hydrocarbon (HC) or CO contained in the exhaust gas of an automobile or the like. For example, an oxide semiconductor (n type) such as SnO.sub.2 or the like is used as a sensing element for measuring the concentration of a combustible gas component such as HC or CO. Specifically, oxygen in the atmosphere adsorbs onto the sensing element through an effect induced by negative charges. When the atmosphere contains a combustible gas component such as HC or CO, the combustible gas component undergoes a combustion reaction with the adsorbed oxygen, thereby causing oxygen to be desorbed from the sensing element. Because a change in electric resistance of the sensing element associated with the oxygen desorption depends on the combustible gas component concentration of the atmosphere, the combustible gas component concentration of the atmosphere can be obtained by measuring the change in electric resistance. However, such a resistance-type sensor has a drawback in that an output from the sensing element formed of an oxide semiconductor is likely to vary depending on the concentration of oxygen or water vapor contained in the exhaust gas. Accordingly, even when the combustible gas component concentration remains unchanged, the sensor output value indicative of the combustible gas component concentration varies depending on, for example, the oxygen concentration of the exhaust gas.
In order to solve the above problem, an apparatus for measuring a combustible gas component concentration having the following structure is disclosed in Japanese Patent Application Laid-Open No. 8-247995. In this apparatus, the sensing element has two processing zones. An exhaust gas is introduced into a first processing zone via a first diffusion-controlling means. Oxygen is pumped out from the first processing zone by means of a first oxygen pumping element so as to reduce the oxygen concentration of the first processing zone to a low value at which combustible gas components are not substantially burned. Next, the gas having the thus-reduced oxygen concentration is introduced into a second processing zone via a second diffusion-controlling means. Oxygen is pumped into the second processing zone by means of a second oxygen pumping element so as to burn the combustible gas component. The combustible gas component concentration is determined based on the value of current flowing through or voltage developed across the second oxygen pumping element.
However, in the apparatus disclosed in the above-described patent publication, the second oxygen pumping element is operated such that the oxygen concentration of the second processing zone falls within a certain constant range. Accordingly, in addition to the second oxygen pumping element, the use of an element (for example, an oxygen concentration cell element) for measuring the oxygen concentration of the second processing zone is substantially unavoidable. Accordingly, the number of required elements for the second processing zone increases, thereby resulting in a complicated sensor structure and an increase in manufacturing cost.
Also, in the above disclosed apparatus, the oxygen concentration of the exhaust gas introduced into the first processing zone is reduced by means of the first oxygen pumping element to "a low value at which a combustible gas component is not substantially burned". According to this publication, the low value is not higher than 10.sup.-14 atm, preferably not higher than 10.sup.-16 atm, and is normally about 10.sup.-20 atm. However, when the oxygen concentration of the first processing zone is set at such a low value, the following problem arises related to accuracy in measuring the combustible gas component concentration.
Specifically, an exhaust gas generally contains a fair amount of water vapor in addition to combustible gas components such as hydrocarbon, carbon monoxide and hydrogen. Generally, the amount of water vapor varies according to operating conditions of an internal combustion engine. According to studies conducted by the present inventors, when the oxygen concentration of such an exhaust gas is reduced to the above-mentioned value, a portion of the water vapor is decomposed into hydrogen and oxygen. The thus-generated oxygen is pumped out from the first processing zone by means of the first oxygen pumping element, whereas the thus-generated hydrogen is not pumped out, but introduced into the second processing zone where the hydrogen induces combustion. When the gas to be measured (also referred to as an object gas) contains a combustible gas component composed primarily of hydrocarbon, the combustion of hydrogen generated by the decomposition of water vapor significantly affects accuracy in measuring the hydrocarbon concentration. Notably, the measurement examples disclosed in the above patent publication were all conducted under conditions where the water vapor concentration of the object gas was constant, and did not take into account the influence of a variation in water vapor concentration when measuring a combustible gas component concentration.
As disclosed in the above patent publication, a proton pump may be additionally used in order to pump out the thus-generated hydrogen from the first processing zone, so that only HC is selectively burned to thereby improve measurement accuracy. However, this method merely employs the proton pump as a means of last resort for coping with hydrogen generation associated with decomposition of water vapor. Addition of the proton pump makes the sensor structure and sensor control mechanism complex, causing an increase in apparatus cost. Furthermore, residual hydrogen which the proton pump has failed to pump out may induce a measurement error.
Also, the following problem is encountered. With the recent tendency of tightening exhaust gas regulations for air pollution control, internal combustion engines such as gasoline engines, diesel engines and the like engines tend to shift to lean-burn operation in order to suppress generation of HC associated with incomplete combustion. An exhaust gas produced under lean-burn conditions has an oxygen concentration higher than that produced under stoichiometric or rich conditions. When the above-described conventional apparatus is applied to such an exhaust gas, an oxygen pumping element is significantly burdened in order to reduce the oxygen concentration to the above-mentioned low value. As a result, the service life of the oxygen pumping element is shortened. Furthermore, since the operating power of the oxygen pumping element must be increased, a high output peripheral control circuit must also be used which in turn increases the apparatus cost.